Stainless steel oil feeding pipe

ABSTRACT

A fuel-filler tube, which is made of a welded pipe of corrosion-resistant austenitic or ferritic stainless steel, has a fuel-supply opening with high dimensional accuracy. The austenitic stainless steel has hardness of 180 HV or less with a work-hardening coefficient of 0.49 or less. The ferritic stainless steel has Lankford value of 1.2 or more with elongation of 30% or more by a uniaxial tensile test.

INDUSTRIAL APPLICATION

The present invention relates to a fuel-filler tube for an automobile,which is made of an expanded stainless steel pipe without cracks so asto reserve fuel without diffusion over a long term.

BACKGROUND OF THE INVENTION

A fuel-filler tube for an automobile has been made of a welded steelpipe, and a fuel-supply opening is formed at its end. The fuel-supplyopening is formed by pressing with a punch or bulging process to expandan end of a welded steel pipe, but the steel pipe is often cracked atthe formed part. In this consequence, there is a strong demand forprovision of a welded steel pipe with good formability.

The fuel-filler tube is installed in an automobile, in the state that itis coupled with a fuel tank. If the fuel-filler tube has poorairtightness, evaporated gasoline diffuses to the open air. Diffusion ofgasoline shall be avoided for maintenance of clean atmosphere, but cannot be inhibited by use of a conventional fuel-filler tube made ofsynthetic resin. Another type of a fuel-filler tube, which is made of aplain steel pipe expanded at its end, coated with a chromium layer andfurther coated with a paint film, is not always protected fromcorrosion, when it is exposed to a corrosive atmosphere such as a saltyarea. Corrosion also occurs inside the fuel-filler tube, which isexposed to a corrosive atmosphere containing an organic acid such asdenatured gasoline or alcoholic fuel, and causes occurrence of pittingand opened holes in the end. Consequently, the fuel-filler tubedrastically looses airtightness.

In order to overcome these disadvantages, applicability of stainlesssteel, i.e. a representative corrosion-resistant material to afuel-filler tube, has been researched and examined for maintenance ofairtightness over a long term. Stainless steel is well resistant tocorrosion without necessity of plating or painting, but hard and easilywork-hardened compared with plain steel. Due to the materialisticcharacteristics, a stainless steel pipe is difficult to form to apredetermined shape without cracks at its expanded part.

By the way, fuel-filler tubes, which are made of an expanded steel pipewith small diameter, are sometime used in response to lighteningautomobiles. However, a fuel-supply opening is unchanged in size withabout 50 mm or so in inner diameter, regardless the size of a steelsheet. Since a steel pipe is necessarily formed at its end with greatexpansion ratio in this case, excellent formability of steel material isstrongly demanded.

However, there is no proposal for provision of a stainless steel pipe,which exhibits good expansibility enough to be formed to a product shapeas well as corrosion-resistance necessary for the purpose.

SUMMARY OF THE INVENTION

The present invention aims at provision of a stainless steel fuel-fillertube, which has good corrosion-resistance and has a fuel-supply openingformed with high dimensional accuracy, by selecting a kind of austeniticstainless steel on the basis of hardness and work-hadenability or a kindof ferritic stainless steel on the basis of Lankford value (r-value).

In an austenitic-type fuel-filler tube, an austenitic stainless steelsheet with hardness of 180 HV or less and a work-hardening coefficient(n-value) of 0.49 or less is selected and processed to a pipe

In a ferritic-type fuel-filler tube, a ferritic stainless steel sheetwith elongation of 30% or more by a uniaxial tensile test and Lankfordvalue of 1.2 or more is selected and processed to a pipe.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a graph showing hardness of an expanded part in every formingstep, when an end of a steel pipe is formed to a shape of a fuel-supplyopening by a multi-stepped process.

FIG. 2 is a graph showing change of a load applied to a steel pipe alongan axial direction in every forming step.

DETAILED DESCRIPTION OF THE INVENTION

Since a stainless steel is harder material than plain steel, a biggerload is necessary for expanding a welded pipe of stainless steel, and astainless steel pipe is often buckled as increase of an expansion ratio.The forming load becomes bigger and bigger as advance of steps during amulti-stepped forming process, since stainless steel sheet is easilywork-hardened. Although buckling of a stainless steel pipe is avoided byincreasing number of forming steps with a small expansion ratio at eachstep, increase of forming steps complicates a manufacturing process andraises a manufacturing cost. Moreover, when a formed stainless steelpipe is work-hardened above 500 HV, it is hardly plastically deformedany more and easily cracked at its expanded end during the followingforming step. Especially, a ferritic stainless steel pipe is oftencracked at its expanded end, since its elongation and Lankford value arelower than a plain steel pipe.

The inventors have researched effects of physical properties of astainless steel sheet used as material for a fuel-filler tube onexpanding.

Austenitic stainless steel is the material, which is easily hardenedcompared with plain steel, due to transformation of matrix tostrain-induced martensite caused by plastic deformation. Even if it issoft initially the work-hardened state requires a big forming load inthe following work step, and it is often cracked and buckled due toincrease of the forming load. The hardening tendency of the austeniticstainless steel sheet originated in generation of strain-inducedmartensite is represented by a work-hardening coefficient (n-value).

Initial hardness at a high-level means difficult formation of a weldedpipe and requires a big forming load. In this case, an expansion ratiois inevitably determined at a low value in order to inhibit buckling ofa stainless steel pipe or its sintering with a punch. Since a stainlesssteel pipe generally has poor ductility as increase of initial hardness,it is easily cracked during forming.

In this point of view, the inventors have investigated hardness and awork-hardening coefficient (n-value) of a starting material forsearching a stainless steel sheet suitable for expanding to shape of afuel-filler tube, and discovered that an austenitic stainless steelsheet with hardness of 180 HV or less and a work-hardening coefficient(n-value) of 0.49 or less is suitable as a starting material formanufacturing a fuel-filler tube with relatively small number of formingsteps with a small forming load at each step. The work-hardeningcoefficient (n-value) is measured by a tensile test as follows: Astainless steel sheet is sampled along its rolling direction, shaped toa test piece No. 13B regulated under JIS Z2201 and tensioned. A curve oftrue tensile strain with logarithmic elongation is drawn from the testresults to calculate a gradient of the curve as a work-hardeningcoefficient (n-value). As n-value is bigger, a stainless steel isregarded as a material easier to be work-hardened.

On the other hand, ferritic stainless steel is harder with lowerelongation than plain steel, due to high Cr content. However, in thecase where a pipe is expanded under application of a tensile stressalong a circumferential direction and a compression stress along anaxial direction, improvement of expansibility can not be expectedbecause of poor ductility of stainless steel.

Lankford value (r-value) is useful for evaluating metal flow along anaxial direction with small reduction of thickness. In this regard, theinventors have researched various ferritic stainless steel sheets to bewell formed to a product shape, and discovered that a ferritic stainlesssteel sheet, which has elongation of 30% or more and Lankford value(r-value) of 1.2 or more, is an optimum material to be formed to apredetermined shape having a fuel-supply opening at its end withoutoccurrence of cracks or other defects. Lankford value (r-value) ismeasured by tensile test as follows: A ferritic stainless steel sheet issampled along its rolling direction, shaped to a test piece No. 13Bregulated under JIS Z2201 and tensioned at a rate of 20 mm/minute.Thickness and width of the test piece are measured after application of15% tension strain, and natural logarithm of reduction rate of width isdivided by natural logarithm of reduction rate of thickness to calculatea quotient regarded as Lankford value (r-value). Furthermore, the testpieces are tensioned until fracture, and the fractured parts are buttedtogether to measure elongation between predetermined marks. The measuredelongation is regarded as fracture elongation.

When an austenitic stainless steel sheet with hardness of 180 HV or lessand a work-hardening coefficient (n-value) of 0.49 or less is used as amaterial for a fuel-filler tube, it can be formed with highexpansibility. Therefore, a welded steel pipe, even which is small insize, can be expanded to a product shape having a fuel-supply openingwith a predetermined size. When a ferritic stainless steel sheet withelongation of 30% or more and Lankford value (r-value) of 1.2 or more isused as a material for a fuel-filler tube, it is also expanded to aproduct shape having a fuel-supply opening with a predetermined size.The welded pipe is manufactured from any of the stainless steel sheetsby sizing the stainless steel sheet to predetermined width, forming thesized sheet to a cylindrical shape, and arc-, laser- orresistance-welding both sides of the sheet together. A seamless pipe isalso expanded to a fuel-filler tube having a fuel-supply opening at itsend, as far as its hardness and work-hardening coefficient (n-value) areless than 180 HV and 0.49, respectively.

The other features of the present invention will be more apparent fromthe following examples with the drawings, although the examples do notlimit scope of the present invention.

EXAMPLE 1

Welded pipes of 25.4 mm in outer diameter, 0.5 mm in thickness and 350mm in length were manufactured from several stainless steel sheets shownin Table 1. An expanded part of 51.4 mm in outer diameter was formed atan end of each welded pipe by repetition of expanding, in order toresearch effects of hardness and a work-hardening coefficient (n-value)on formability of the welded pipe.

A welded pipe L was easily buckled during expanding, since it was hard(185 HV) with a large work-hardening coefficient of 0.52. Its formingprocess was necessarily divided to many steps with a small expansionratio, but an outer diameter of the expanded part was 42.4 mm at most.Another welded pipe M of a ferritic stainless steel was formed with alarge reduction of thickness, since it had poor ductility withelongation of 28% and Lankford value of 1.16. Therefore, its expansionratio without cracks was 42.4 mm at most.

On the other hand, any welded pipe, which was made of a austeniticstainless steel with controlled hardness and work-hardening coefficient,was expanded to an outer diameter of 51.4 mm (in other words, asufficient inner diameter for a fuel-supply opening) at its end.Especially, a welded pipe of a Cr—Ni austenitic stainless steelcontaining Cu was formed to an objective outer diameter in five steps,and also formed to 53.0 mm in outer diameter without cracks. Weldedpipes of ferritic stainless steels with controlled elongation andLankford value were also formed to an outer diameter of 51.4 mm withoutcracks.

TABLE 1 Stainless Steels Used In Example 1 With Properties ComparativeNote Inventive Examples Examples Kind Of Welded Pipe A B C D E F G H I JK L M Alloying Elements C 0.06 0.02 0.05 0.01 0.01 0.03 0.03 0.01 0.020.01 0.01 0.06 0.01 (mass %) Si 0.6 0.4 0.6 0.3 0.3 0.3 0.4 0.2 0.560.04 0.2 2.5 0.2 Mn 1.3 1.4 1.0 1.6 1.5 1.4 0.3 0.2 0.2 0.2 0.2 0.8 0.2Ni 8.5 9.1 10.1 7.9 12.0 4.67 — — — — — 9.4 — Cr 18.2 18.7 17.0 16.917.1 16.8 16.5 18.0 19.2 18.9 22.1 17.8 30.2 Cu — — — 3.2 2.0 3.82 — —0.5 0.5 — — — Mo — — 2.1 — — — — 1.0 — — 1.2 — 2.1 Ti — — — — — — 0.30.3 — — 0.2 — 0.2 Nb — — — — — — — — 0.5 0.5 0.2 — 0.2 MechanicalProperties 0.2% Yield Strength 284 270 314 188 176 241 285 270 360 274343 285 431 (N/mm²) Tensile Strength 659 570 578 479 476 529 480 440 530444 510 710 568 (N/mm²) Elongation (%) 60 60 55 61 55 54 32 34 31 34 3064 28 Hardness HV 160 154 156 119 109 127 154 144 177 139 166 185 192n-value 0.49 0.48 0.49 0.41 0.38 0.43 0.23 0.22 0.20 0.21 0.18 0.52 0.23r-value — — — — — — 1.8 1.25 1.76 1.84 1.31 — 1.16

TABLE 2 Occurrence Of Cracks And Reduction Of Thickness At Every FormingStep Note Kind Of Welded Pipe Comparative Forming Outer diameterInventive Examples Examples Step of formed part Evaluation A B C D E F GH I J K L M 1^(st) 30.0 mm (1) — — — — — — — — — — — ◯ — (2) 2^(nd) 31.2mm (1) ◯ ◯ ◯ — — — ◯ — ◯ — ◯ ◯ ◯ (2) 3^(rd) 33.4 mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ (2) 10.4 10.2 10.6  8.0  7.2  8.5  5.7  5.3  6.2  5.1  7.911.1  8.7 4^(th) 36.5 mm (1) ◯ ◯ ◯ — — — — — — — — ◯ — (2) 5^(th) 40.0mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ (2) 6^(th) 42.4 mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ (2) 18.2 17.5 17.8 13.7 12.0 14.2 11.9 11.7 12.0 11.2 13.221.2 18.7 7^(th) 44.0 mm (1) ◯ ◯ ◯ — — — — — — — — X X (2) 8^(th) 48.5mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ (2) 9^(th) 51.4 mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯◯ (2) 24.2 23.2 23.7 19.1 18.1 18.7 20.3 19.8 20.6 19.3 21.5 10^(th )53.0 mm (1) X X X ◯ ◯ ◯ X X X X X (2) Evaluation (1): Absence (◯) orpresence (X) of cracks Evaluation (2): reduction of thickness (%)

Sectional hardness of an expanded part of each welded pipe was measuredat every forming step, so as to research change of hardness in advanceof forming steps. Results are shown in FIG. 1. It is noted that a weldedpipe L was excessively hardened up to 550 HV when its end was expandedto 42.4 mm in outer diameter.

On the other hand, welded pipes A and D were slightly hardened to 460 HVand 315 HV, respectively, although they were made of the same type ofaustenitic stainless steel. These results prove that the austeniticstainless steels for the welded pipes A and D were materials hardlywork-hardened. Another welded pipe J of a ferritic stainless steel wassofter in a expanded state compared with austenitic stainless steel,since its work-hardening coefficient was low of 0.21.

Furthermore, a high-frequency welded pipe of 25.4 mm in outer diameter,1.0 mm in thickness and 350 mm in length was expanded to an outerdiameter of 52.4 mm at its end in three steps shown in Table 3. A weldedpipe, which as made of any stainless steel with hardness and awork-hardening coefficient controlled according to the presentinvention, was expanded to an outer diameter of 52.4 mm without cracksor buckling. Reduction of thickness was small enough to use as afuel-supply opening with good properties. But, welded pipes L and M wereruptured at their ends during the third step.

TABLE 3 Occurrence Of Cracks And Reduction Of Thickness At Every FormingStep Note Kind of welded pipe Comparative Forming Outer diameterInventive Examples Examples Step of formed Part Evaluation A B C D E F GH I J K L M 1^(st) 34.4 mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ (2) 10.6 10.411.0  9.0  8.1  9.4  7.8  7.5  8.1  7.4  8.5 12.1  9.5 2^(nd) 43.4 mm(1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ (2) 18.8 18.2 18.5 15.8 13.9 16.2 14.013.9 14.1 13.2 15.6 22.3 19.8 3^(rd) 52.4 mm (1) ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ XX (2) 25.1 24.1 24.3 20.5 19.8 21.1 21.5 20.3 21.9 20.1 22.3 Evaluation(1): Absence (◯) or presence (X) of cracks Evaluation (2): reduction ofthickness (%)

Furthermore, a load applied to each welded pipe along its axialdirection was measured at every forming step. Results shown in FIG. 2prove that a load applied to the welded pipe, which was made of anaustenitic stainless steel defined by the present invention, wasrelatively small. When an expanded part of a welded pipe J made of aferritic stainless steel hardly work-hardened was further formed, a loadapplied along an axial direction was held at a lower level compared withaustenitic stainless steel. Decrease of the load means suppression ofheat generated during forming and inhibits sintering a welded pipe witha punch. As a result, a lifetime of a punch was prolonged, and thewelded pipe was also formed to a product shape having an inner surfacefree from defects.

INDUSTRIAL ADVANTAGES OF THE INVENTION

The fuel-filler tube according to the present invention asabove-mentioned is made from a welded pipe of an austenitic stainlesssteel sheet with hardness of 180 HV or less and a work-hardeningcoefficient (n-value) of 0.49 or less or a ferritic stainless steel withelongation of 30% or more by a uniaxial tensile test and Lankford value(r-value) of 1.2 or more. Since the welded pipe can be expanded to aproduct shape having an expanded fuel-supply opening at its end withoutcracks or buckling even under severe conditions. A welded pipe of smalldiameter can be also formed to a product shape having a fuel-supplyopening at its end with high dimensional accuracy, with high expansionratio. Consequently, a fuel-filler tube in small size is provided as alightened corrosion-resistance part for an automobile.

1. An austenitic stainless steel fuel-filler tube having a fuel supplyend with an exnanded portion made from a pipe of an austenitic stainlesssteel with hardness of 180 HV or less and a work-hardening coefficient(n-value) of 0.49 or less, said expanded portion formed by multiplepunch pressing steps.
 2. A ferritic stainless steel fuel-filler tubehaving a fuel supply end with an expanded portion made from a pipe offerritic stainless steel with elongation of 30% or more by a uniaxialtensile test and Lankford value (r-value) of 1.2 or more said expandedportion formed by multiple punch pressing steps.